Expansion system in the heat-transfer-medium circuit of a solar-thermal power plant

ABSTRACT

In an expansion system ( 11 ) of the heat transfer medium circuit ( 1 ) of a solar thermal power plant, comprising a plurality of expansion tanks ( 12   a,    12   b,    12   c ) and/or flooding tanks which are arranged in the heat transfer medium circuit ( 1 ), a solution is to be created which enables a technically simplified and constructionally less costly compensating or expansion system to be created. This is achieved by the expansion system ( 11 ) comprising a plurality of expansion tanks ( 12   a,    12   b,    12   c ) which are arranged at basically the same height level and are in fluid-conducting communication.

The invention is directed towards an expansion system of the heat transfer medium circuit of a solar thermal power plant, comprising a plurality of expansion tanks and/or flooding tanks which are arranged in the heat transfer medium circuit.

Furthermore, the invention is directed towards a solar thermal power plant with a solar field which is formed from parabolic trough collectors.

Solar thermal power plants have a heat transfer medium circuit and a water/steam cycle which is in functional communication therewith. A heat transfer medium or heat exchanging medium is circulated in the heat transfer medium circuit, wherein in absorber pipes arranged in the focal line of parabolic trough mirrors the medium is conducted through a solar field and is heated there by rays of the sun which fall onto the parabolic trough mirrors and are reflected thereupon. This heat which is concentrated by the sun upon the absorber pipe in parabolic trough collectors, having the parabolic trough mirrors, of the solar thermal power plant is given off via heat exchangers to the water/steam cycle in which steam is produced. Via a turbine, the heat which is contained in the steam is converted into useful energy, for example into electric energy via a generator which is connected to the turbine. In solar thermal power plants with parabolic trough mirrors, a thermal oil is customarily used as heat transfer medium or heat exchanging medium. On account of the temperature difference in the heat transfer medium circuit which occurs during operation of such a solar thermal power plant, for example when changing from day to night, and, associated therewith, on account of varying volumes of heat transfer medium which for example is hotter during the day than at night, provision must be made for a compensating or expansion system which balances out and compensates the volumetric expansions of the heat transfer medium circuit in the case of increased temperature.

It is known from experience to design an expansion system consisting of an elevated expansion tank which is arranged several meters above ground level and with a plurality of flooding tanks which are arranged at a level beneath it and are in fluid-conducting communication with the expansion tank, wherein, depending on the size of the power plant, a plurality of flooding tanks are provided. A gas cushion of nitrogen (N₂) is formed in the expansion tank and is located above the fluid level of the heat transfer medium in the expansion tank. With regard to level, the known expansion tank is arranged at the highest point of the heat transfer medium circuit. The expansion tank is provided with a vent valve in order to be able to blow off developing gas portions, for example oil vapor, of the heat transfer medium. Via pipes, in which a system of pumps and valves is arranged, the expansion tank is connected to the flooding tanks in order to be able to adapt an adjustment of the filling level of the expansion tank to the varying operating conditions. Furthermore, the pumps and valves are designed with a redundancy feature so that the operating reliability of the expansion system is ensured.

A disadvantage with this prior art is the high investment cost associated therewith, which arises from the fact that the flooding tanks and the expansion tank have to be arranged at different heights above ground level so that the heat exchanging system, of which the expansion system is a component part, has to be constructed in at least two tiers at different heights.

In addition, the valves and pumps have to be provided in double the number. This also leads to the internal energy consumption of the power plant being relatively high since the pumps which are provided with the redundancy feature consume electric power. Finally, an (increased) risk of failure and corresponding maintenance and repair expenditure are also associated with the presence of pumps and valves. Also, the entire system must be incorporated into the plant control system of the solar thermal power plant.

The invention is therefore based on the object of creating a solution which enables a technically simplified and constructionally less costly expansion system to be created.

In an expansion system of the type referred to in more detail in the introduction, this object is achieved according to the invention by the expansion system comprising a plurality of expansion tanks which are arranged basically at the same height level and are in fluid-conducting communication with each other.

In a solar thermal power plant of the type referred to in more detail in the introduction, this object is achieved according to the invention by it having at least one expansion system according to one of claims 1-10.

On account of the design according to the invention of an expansion system, it is no longer necessary to design the heat transfer medium circuit, and especially the expansion system, in two tiers consisting of flooding tanks with an expansion tank which is arranged above them. This reduces the investment costs and the constructional expenditure considerably. Furthermore, the expansion tanks are in fluid-conducting communication with each other so that without pumps or other valves the fluid can flow from one expansion tank independently into an adjacent expansion tank, wherein the fluid-conducting connections are formed below the fluid level in the individual expansion tanks and consequently the same fluid level of heat transfer medium is established overall in all the expansion tanks across the number of expansion tanks which are provided in each case. In this way, pumps and valves between the expansion tanks are no longer necessary, which significantly reduces maintenance and repair operations. In particular, movable parts between two expansion tanks are no longer necessary. In addition to the reduced investment cost and the reduced maintenance and repair expenditure, the internal consumption of electric energy is also reduced and the plant control system of the solar thermal power plant is unloaded. In principle, the expansion system according to the invention comprises at least one expansion tank, preferably a plurality of expansion tanks, which is, or are, mounted at ground level and in fluid-conducting communication with the heat transfer medium circuit.

In an advantageous embodiment, the invention provides that at least one expansion tank is in fluid-conducting communication with a cross-sectionally widened section of a pipe of the heat transfer medium circuit which conducts the heat transfer medium. The forming of a cross-sectionally widened section inside the pipe which circulates the heat transfer medium enables degassing of, for example, oil vapor which develops in the expansion tanks since in the widened pipe section the flow velocity of the heat transfer medium is reduced and therefore discharge of gas from the fluid flow is made easier.

It is advantageous for achieving a satisfactory discharge of gas according to an embodiment of the invention, furthermore, if the pipe section opens into an expansion tank.

In order to realize a transition—which impedes the flow of heat transfer medium as little as possible and enables a satisfactory degassing of the heat transfer medium—from the normal cross section of the pipe which conducts the heat transfer medium to the pipe section which is cross-sectionally widened or cross-sectionally enlarged in relation to it, the invention is furthermore distinguished by the fact that the cross-sectionally widened pipe section, at least in the upstream direction of the heat transfer medium circuit, has a transition section, preferably with a uniformly constant change of cross section, which opens into the normal cross section of the pipe which conducts the heat transfer medium. As a result of this, a uniform reduction of the flow velocity can be achieved. In order to also achieve on the other hand an equally uniform increase of the flow velocity of the heat transfer medium when leaving the cross-sectionally widened pipe section, the invention furthermore provides that the cross-sectionally widened pipe section, in the upstream direction and in the downstream direction of the heat transfer medium, has in each case a transition section, preferably with a uniformly constant change of cross section, which opens into the normal cross section of the pipe which conducts the heat transfer medium.

It is also advantageous in an embodiment of the invention, furthermore, if the expansion tanks are pressurized with a gas cushion of equal pressure formed from the same protective gas.

In this case, the pressure expediently also lies above the vapor pressure of the heat transfer medium, which the invention also provides. This has the advantage that evaporation of the heat transfer medium is prevented as a result.

The number of expansion tanks with which the expansion system is equipped depends individually upon the respective plant size, and by the same token the size of the individual expansion tanks is designed depending upon the design and capacity of the heat transfer medium circuit and of the solar thermal power plant. In this case, the size of the individual expansion tanks is basically limited by the respectively selected production processes and also transporting procedures.

In a development, the invention additionally provides that at least one expansion tank has a safety valve in the form of a pressure relief valve. As a result of this, a pressure rise which exceeds the permissible amount, and therefore an excessive accumulation of gas in the expansion tanks which are interconnected in the form of communicating pipes, are avoided.

In order to be able to undertake degassing of the expansion tanks depending upon desire and requirement, the invention furthermore provides that at least one expansion tank has a degassing valve. If the gas chambers or gas cushion chambers of the individual expansion tanks are interconnected in fluid-conducting communication via a pipe in which the degassing valve is arranged, then, from a control room, for example, the degassing valve can be operated as a blow-off valve and in this way a uniform degassing of all the expansion tanks which are provided and in fluid-conducting communication with the degassing valve can be carried out.

The invention therefore also provides that the degassing valve can be controlled, via a signal line, from a control room.

Finally, the invention in the embodiment of the solar thermal power plant is distinguished by the fact that it exclusively has an expansion system, or a plurality of expansion systems, according to one of claims 1-10.

The invention is exemplarily explained in more detail below with reference to a drawing. This shows in a schematic representation the heat transfer medium circuit 1 of a solar thermal power plant, which circuit is represented schematically and symbolized by a pipeline 2 which is routed in a circular manner. Formed in the heat transfer medium circuit 1 is the solar field 3, equipped with parabolic trough collectors, in which the heat transfer medium 6, which is moved in the heat transfer medium circuit 1 by a pump 5, or a plurality of pumps, in the flow direction 4, absorbs reflected solar radiation, reflected by the mirror surface of the parabolic trough collectors, in the form of heat. The heated heat transfer medium 6 gives off heat to the water/steam cycle 8 of the solar thermal power plant in a heat transfer stage 7 which has a plurality of heat exchangers. Downstream of the heat transfer stage 7 and upstream of the pump 5, a cross-sectionally widened section 10 is then formed in a pipe 9 of the heat transfer circuit 1. In a way which is not apparent from the figure, a transition section is formed at both ends of the cross-sectionally widened section 10 in each case, along which transition section is formed a constant change of cross section from the cross section of the cross-sectionally widened pipe section 10 to the cross section of the pipe 9 which conducts the heat transfer medium 6 in this region so that this transition section from the cross-sectionally widened pipe section 10 opens into the normal cross section of the pipe 9. The cross-sectionally widened pipe section 10 is a component part of the expansion system which is represented as a dashed line 11. In the exemplary embodiment, this expansion system 11 comprises three expansion tanks 12 a, 12 b, 12 c, of which the middle expansion tank 12 b is in fluid-conducting communication with the cross-sectionally widened pipe section 10, as is indicated by arrows 13 a, 13 b. The cross-sectionally widened pipe section 10 preferably opens directly into an expansion tank, this being the expansion tank 12 b in the exemplary embodiment.

The expansion tanks 12 a, 12 b and 12 c, at least approximately, are basically positioned at the same height and in their lower regions are interconnected in each case by fluid pipes 14 a, 14 b so that a volume of heat transfer medium 6 forms and collects in each case inside the expansion tanks 12 a, 12 b, 12 c in such a way that the same level of the fluid surface is achieved in all three expansion vessels. As a result of the design of the fluid-conducting connection—which is such that it is an intercommunicating connection—between the individual expansion tanks 12 a, 12 b, 12 c, it is no longer necessary to design and to arrange pumps or valves between these expansion tanks and in the fluid pipes 14 a, 14 b. The exchange of fluid between the individual expansion tanks is carried out automatically. Above the fluid surface level, each of the expansion tanks 12 a, 12 b, 12 c is equipped with a gas cushion consisting of protective gas, for example nitrogen in the exemplary embodiment. In this case, the pressure in each of the expansion tanks 12 a, 12 b, 12 c is equal and the same protective gas is also provided in each expansion tank. Also, the chambers 15 a, 15 b, 15 c which have the gas cushions are in fluid-conducting communication with each other via pipes 16 a, 16 b, 16 c. A vent valve 17 is arranged in the pipes 16 a, 16 b, 16 c and, in a way not shown, is in functional communication, via a signal line, with a control center or control room, from which control room the vent valve can be controlled and, depending upon the desired degassing, blowing off of gas from the gas cushions of the expansion tanks 12 a, 12 b, 12 c is possible as a result.

For safety reasons, at least one of the expansion tanks 12 a, 12 b, 12 c is also equipped with a safety valve 18 which is formed as a pressure relief valve which blows off gas from the expansion system 11 in the event of impermissibly high gas pressure.

Also, if in the exemplary embodiment the expansion system 11 comprises three expansion tanks 12 a, 12 b, 12 c, then this, however, can comprise any number of expansion tanks, but at least one which is selected and designed depending upon the size and design of the heat transfer medium circuit 1 and of the water/steam cycle and upon the capacity of the solar thermal power plant.

The expansion tanks 12 a, 12 b, 12 c are mounted at ground level with a drop to the pipe section 10 in order to prevent dry running of the pump(s) 5 and therefore to be easily reachable for maintenance and/or repair purposes. 

1. An expansion system of a heat transfer medium circuit of a solar thermal power plant, comprising a plurality ⁻of expansion tanks and/or flooding tanks which are arranged in the heat transfer medium circuit, wherein the expansion system comprises a plurality of expansion tanks which are arranged at basically the same height level and in fluid-conducting communication with each other.
 2. The expansion system as claimed in claim 1, wherein at least one expansion tank is in fluid-conducting communication with a cross-sectionally widened section of a pipe of the heat transfer medium circuit which conducts heat transfer medium.
 3. The expansion system as claimed in claim 2, wherein the cross-sectionally widened pipe section opens into an expansion tank.
 4. The expansion system as claimed in claim 2, wherein the cross-sectionally widened pipe section, at least in the upstream direction of the heat transfer medium circuit, has a transition section, which opens into the normal cross section of the pipe which conducts the heat transfer medium.
 5. The expansion system as claimed in claim 2, wherein that the cross-sectionally widened pipe section, in the upstream direction and downstream direction of the heat transfer medium circuit, has in each case a transition section which opens into the normal cross section of the pipe which conducts the heat transfer medium.
 6. The expansion system as claimed in claim 1, wherein the expansion tanks are pressurized with a gas cushion of equal pressure formed from the same protective gas.
 7. The expansion system as claimed in claim 6, wherein the pressure of the gas cushions lies above the vapor pressure of the heat transfer medium.
 8. The expansion system as claimed in claim 1, wherein at least one expansion tank has a safety valve in the form of a pressure relief valve.
 9. The expansion system as claimed in claim 1, wherein at least one expansion tank has a degassing valve.
 10. The expansion system as claimed in claim 9, wherein the degassing valve can be controlled, via a signal line, from a control room.
 11. A solar thermal power plant with a solar field formed from parabolic trough collectors, wherein it has at least one expansion system as claimed in claim
 1. 12. The solar thermal power plant as claimed in claim 11, wherein it exclusively has an expansion system, or a plurality of expansion systems, as claimed in claim
 1. 13. The expansion system as claim in claim 4, wherein the transition section of the cross-sectionally widened pipe section has a uniformly constant change in cross-section.
 14. The expansion system as claim in claim 5, wherein each transition section of the cross-sectionally widened pipe section has a uniformly constant change in cross-section. 